Device for producing sustained magnetic self-focusing streams



p 2, 1959 w. H. BENNETT 2,905,842

DEVICE FOR PRODUCING SUSTAINED MAGNETIC SELF-FOCUSING STREAMS Filed Nov. 2 2, 1957 r 27 K D C D C SOURCE SOURCE P SE STEADY COMPONENT INTER PULSE TIME INVENTOR WILLARD H. BENNETT.

. a BY ATTORNEYJ vice of} the present invention;

DEVICE FOR PRODUCING SUSTAINED lVIAG- NETIC SELF-FOCUSING STREAMS.

Willard H. Bennett, Washington, D.C.

Application November 22, 1957, Serial No. 698,315 6 Claims. or. 313-62 (Granted under Title 35, US. Code (1952),v sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States from their circular orbit to produce a desired result such as bombarding a target to produce X-rays. It has also been proposed that a self-focusing stream be produced in a closed loop in a betatron, wherein motions of the electrons in a self-focusing stream in directions transverse to the axis of the stream would produce radiation which would damp such motion and that such damping would oppose the thermal dispersion of the stream. Such a stream would involve the simultaneous acceleration of all of the electrons in the stream and have one major difficulty. The combined action of the accelerating electric field and the self-magnetic field of the current in the stream drives the electrons toward the axis. The continued application of the accelerating electric force produces a continually increasing density of electrons with their associated space-charge neutralizing ionsnear the axis, and the rate at which energy must be supplied to compensate the loss of energy of the electrons due to their collisions with the ions increases correspondingly. Such a rate may amount to thousands of kilowatts.

It is accordingly, an object of the present invention to provide a device for producing self-focusing streams of relativistic electrons in-a closed loop.

Another object is to provide a device which is capable of holding electrons in a closed loop orbit sufliciently long to prevent the electrons from escaping from the guide field.

Still another object is to provide a device which is capable of intensifying the current within a closed loop suflicient to produce self-focusing streams.

Other objects and advantages of the present invention will hereinafter become more fully apparent from the following description of the annexed drawings; which illustrates the preferred embodiments, and wherein:

Big; 1 schematically illustrates an end view of the de- Fig. 2 illustrates a section through the discharge chamherofthe device; i

Fig. 3' illustrates another section through the chamber which illustrates the electron injection tube and the electron path;

Fig. 4 is another section through the chamber which illustrates soft iron members on the outer surface thereof; Fig. 5"represents the steady'rnagnetic guide-field-wit additional sawtooth pulses added thereto; and

Unite States Patent -'Fig; Gillus'trates a-modification ofthe device of -Fig.' 1.

ice

The device of the present invention comprises a cylindrical or pill box shaped evacuated discharge chamber positioned within a magnetic guide-field similar in form to that of a cyclotron. Electrons from an electron linear accelerator are injected in pulses at full energy into the chamber within'the guide-field. The electrons injected ineach pulse are held by an increase in the guide-field lasting long enough for the electrons to radiate energy and shrink inloop radius sufficient to prevent the electrons from escaping from the chamber. By successively injecting electrons into the chamber, the cycling current is built up to the minimum critical value for self-focusing and held there by the guide-field.

Now referring to the drawings wherein like reference characters represent like parts throughout, there is shown by illustration inFig. 1, a device according to the present invention. The device includes a magnetic guidefield similar to that of a cyclotron and which is made to decrease in magnitude with increasing radius at a slow rate. The magnetic field structure is made up from steel laminations or any other suitable material of appropriate c'ontour which includes a pair of cylindrical pole pieces 15 and 16 separated by an air gap within which a cylindrical or pill box shaped discharge chamber 17 is placed. Yoke member 18 completes the magnetic circuit for the flux set 'up in the cylindrical pole pieces. The cylindrical pole pieces 15 and 16 are. surrounded by an annular winding preferably split into two coils 19 which are wound in the same relative direction and connected in series for energization from a. direct current source 21 to maintain the poles steadily magnetized. The spacing between the poles provides a space distribution such as to produce a stabilizing field on charged particles within a chamber 17 as to confine the particles within said chamber to substantially predetermined circular orbits.

The chamber 17 is made of any suitable insulating material such as glass, Sillimanite, porcelain, etc., with side walls 22 supported by a support rod 23 across the center of the chamber which prevents the chamber from collapsing when under a high vacuum. The chamber may be held in position by any means familiar in the art, one of which is illustrated by two pair of supports 24 on the outer circumferential surface and adapted to be positioned within the magnetic guide-field with the lines of force symmetrical with the axis of the chamber. The circumferential surface of the chamber is surrounded by two pair of coils 25 and 26 positioned on opposite sides of holders 24 and each of which are wound in the same relative directions and connected in series for energization respectively from variable direct current sources 27 and 28 which may be the same or different and which can be steeply increased in pulses. The pulsating current in coils 25 and 26 is for the purpose of increasing the flux inside the coils in approximately a sawtooth manner as illustrated by Fig. 5. A pair of coils 31 are provided along the side walls of the chamber and a pulsating current from a source 32' is passed therethrough for the purpose of holding electron orbits in a plane within the chamber and to prevent wobbling along the axis during, operation of the device. An electron linear accelerator 33 of any well known type which periodically injects electrons into the chamber through an electron injection tube 34 is located at the midplane of the chamber. and arranged tangentially to the inner periphery of the chamber. The tangential arrangement permits the electrons to be injected into the chamber such that on entering the chamber the magnetic guidefield will force the electrons into an orbit about the axis of the chamber.

The electron injection tube 34 is surrounded by a magnetiwshielding tube '35 which magnetically shields the beam of electrons from the guide-field until the beam emerges from the injection tube and enters the magnetic the end of the injection tube at 36 while the electrons are rotating within the chamber. The chambr'is also provided with two pieces of soft iron 37 connected with the magnetic pole pieces'and extending over to the center of the chamber at a point approximately 270 degrees from the injection tube. These pieces of soft iron increase the magnetic field nearby and deflect the electron orbits away from the electron injection tube. of the locally increased magnetic field near the soft iron piecs 37 and the locally reduced magnetic field near the magnetic shielding tube operates to set up a field which forces the electrons in orbits towards the center of the chamber such that the electrons do not hit the end of the electron injection tube. The field set up by the soft iron pieces 37 and the magnetic shielding tube 35 will be referred as the piler field;

Fig. 6 illustrates a modification of the coil arrangement about the chamber 17. As shown in Fig; 6, the coils 25 and 26 are placed on the inside of the chamber and the leads brought out through the chamber to the currr ent source. This modification operates in the same manner as that of the coil-chamber arrangement of Fig. l-4.

In operation of the device the cylindrical discharge chamber is positioned in the radially decreasing magnetic field such that the lines of force of the magnetic guidefield are symmetrical with the axis of the chamber. The chamber is evacuated and high energy electronsfrom the electron linear accelerator are injected in pulses into the chamber at an energy large compared to the rest-energy of an electron wherein they are directed in orbits by the guide-field. Beginning near the end of each electron injection pulse a steeply increasing current is'passed through the coils 25 and 26 and then returned slowly to zero at the beginning of the next injection pulse to provide a somewhat sawtooth-shaped increase in the guide-field. The field produced by the steeply increased current will be referred to as the gripper field. The steep rise in the guide-field produced by the gripper field reduces the radii of the orbits of the electrons that were just injected into the chamber, thus holding the electrons and moving them away from the injector and toward the axis of the chamber. The motion of the electrons in approximately circular loops results in the electrons radiating some of their energy, and the reduction in energy produces a corresponding reduction in loop radius. The reduction in gripper field which follows the steep rise in current is made slow enough for the corresponding increase in loop radius of the electrons to remain less thanthe concurrent decrease in loop radius caused by the radiation of electron energy. The injection of pulses of electrons with subsequent application of the 'gripperfield is constantly repeated for continued use and operation of the device. The coils along the side walls of the chamber act on the orbital travel of the electrons to maintain the orbits in a plane such that the electron path does not wobble back and forth along the axis too much; The magnetic field in combination with the gripper field holds the electrons in orbital paths about the axis of the chamber such that they are held in space away from the walls of the chamber.

The first phase build-up of current in the orbits spiralling inward toward the center of the guide-field continues until the total current exceeds the minimum critical current i for self-focusing which can be determinedfrom the relation where c is the speed of light; 6 is the charge on the elec- The combination tron and y is the average energy of an electronidueto momenta transverse to the direction of the stream as seen in the laboratory system of coordinates. The

mean transverse energy 1 is due principally to the injected stream. As these electrons decrease in energy in energy to x from the value at injection of x is determined by V I 21rcx w t For example, electrons injected at 50 mev. with mean angular divergence at injection of one-tenth milliradian, if slowed to 11.4 mev. must build up to a current of 4600 amperes to become self-focusing.

As soon as the critical current is exceeded, the moving electrons will begin to draw together into a self-focusing stream under the effects of the self-magnetic field produced by the electron travel, and the acceleration of the electrons by the self-magnetic field of the stream make the electrons radiate energy of motion transverse to the direction of the stream. This is to be distinguished from the radiation by the electrons due their acceleration i the guide-field. Y

The electrons. are injected into the chamber at full energy and accumulate in the stream without being further energized before the critical current for self-focusing is reached. An application of steady-state self-focusing stream is the use of the stream as a strong magnetic guidefield for ions while the ions are being accelerated to very high energies by means with which the electrons in the stream are not resonant.

Ions, can be injected into the stream to run in the opposite direction around the loop. The ions cannot be stored in the stream by any practical kind of process using radiation like that which was used for storing elec trons because the radiation rate from ions is too small. If the ions are injected so that a part of the first few loops lies inside the concentrated self-focusing stream, coulomb collisions between ions and electrons can deflect a few of the ions through the small angle needed to put those ions in the stream. Those ions will stay in the stream until their transverse energy has become much greater than the ions which have little motion in the direction of the stream. This kind of ion injection can be made rapid enough to keep the stream filled with high energy ions of the species being injected and thus prevent the ions formed by ilonization of residual gas from remaining in the stream and forming any important part of the stream. 7

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.-

What is claimed: v p

1. In a device for producing self-focusing streams of particles comprising a cylindrical chamber adapted to be evacuated and placed within a magnetic field, means for admitting periodic pulses of high speed particles into said chamber, electro-rnagnetic means positioned along the circumference of said chamber and adapted to shrink ,the orbits of particles injected into said chamber and preventing escape from said chamber while subsequent pulses of high energy particles are similarly admitted to :said chamber, and electro-rnagnetic means positioned along the sides of said chamber for holding the orbits of said particles in a plane about the axis of said chamber.

2. A device for producing self-focusing streams of charged particles which comprises an evacuated cylindrical chamber within which charged particles may follow a closed orbital path, means for providing charged particles Within said chamber, means adjacent said chamber for producing a steady magnetic field across said chamber, means for preventing the charged particles in the closed orbits from hitting the end of said means providing said charged particles consisting of means positioned about the circumference of said chamber for superimposing a varying field upon said steady magnetic guide field to shrink the orbital paths of said charged particles and preventing particle escape from said chamber While subsequent pulses of high energy particles are similarly admitted to said chamber.

3. A device as claimed in claim 2 which includes means along the side Walls of said chamber for holding the orbits of said electrons in a plane along the axis of said chamber.

4. A device for producing self-focusing streams of electrons which comprises an evacuated cylindrical chamber within which electrons may follow a closed orbital path, an electron linear accelerator for injecting electrons into said chamber, magnetic pole pieces on opposite sides of said chamber for producing a steady magnetic field with the lines of force symmetrical With the axis of said chamber, means for preventing the electrons in the circular orbits from hitting the end of said electron accelerator, means positioned on the outer circumference of said chamber for superimposing a varying field upon said steady magnetic guide field to shrink the orbital paths of said electrons and preventing particle escape from said chamber While subsequent pulses of high energy particles are similarly admitted to said chamber.

5. A device as claimed in claim 4 wherein the means for superimposing a varying field upon said steady magnetic field is located on the inner circumferential Wall surface of said chamber.

6. A device as claimed in claim 5 Which includes means along the side Walls of said chamber for holding the orbits of said electron in a plane along the axis of said chamber.

References Cited in the file of this patent UNITED STATES PATENTS 2,193,602 Penny Mar. 12, 1940 2,484,549 Blewett Oct. 11, 1949 2,538,718 Wideroe Jan. 16, 1951 2,853,645 Mobley Sept. 23, 1958 

